Stockholders' equity: Common stock buyback program: (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Common stock buyback program:
Authorized amount of common stock repurchases under the Buyback Program	$ 50,000,000
Remaining authorized amount of common stock repurchases under the Buyback Program	$ 43,300,000
Repurchase of common stock (in shares)	0.1	1,200,000	1,700,000
Stock Buyback Amount During the Period	$ 4.5	$ 39,400,000	$ 58,600,000